The primary structure of polymeric materials—that is, the sequential arrangement of monomer units in a polymer chain—is generally poorly controlled in synthetic macromolecules. Common polymers are usually homopolymers, made of the same monomer unit, or copolymers with simple chain microstructures, such as random or block copolymers. These polymers are used in many areas but do not have the structural and functional complexity of defined sequence biopolymers, such as oligonucleotides, nucleic acids, proteins peptides, or oligosaccharides.
There is great utility in defined monomer sequence polymers, i.e. polymers which are assembled from a library of functional building blocks so that the monomer order is exactly defined, and in which at least two or more of the monomers are structurally distinct from each other. For such molecules it may be possible to programme their structural properties, for example folding and self-assembly, and also their macroscopic properties (Lutz J-F et al., “Sequence-Controlled Polymers”, Science 9 Aug. 2013, Vol 341, page 628). Many applications in medicine are also envisaged (Hartmann L and Borner H G, “Precision Polymers: Monodisperse, Monomer-Sequence-Defined Segments to Target Future Demands of Polymers in Medicine” Advanced Materials. 2009, Vol 21, pp 3425-3431).
A key challenge for defined monomer sequence polymers is how to prepare them. Various strategies have been proposed, including biological methods and chemical synthesis using iterative steps in which the monomers are attached one-by-one in a given order. This method suffers from the difficulties of purification at each step. This challenge has been addressed to date (Lutz J-F et al., “Sequence-Controlled Polymers”, Science 9 Aug. 2013, Vol 341, page 628; and Hartmann L and Borner H G, “Precision Polymers: Monodisperse, Monomer-Sequence-Defined Segments to Target Future Demands of Polymers in Medicine” Advanced Materials, 2009, Vol 21, pp 3425-3431) through either advanced polymerisation chemistry or solid phase synthesis as used for sequence defined biopolymers, such as oligonucleotides and peptides.
U.S. Pat. No. 8,664,357 reports a process for use in the preparation of oligonucleotides, peptides and peptide nucleic acids which comprises synthesizing a first compound in a step (i) and then in a step (ii) separating the first compound from a second compound which is a reaction by-product of the synthesis of the first compound and/or an excess of a reagent used for synthesis of the first compound, by a process of diafiltration, where the membrane used for the diafiltration process is stable in organic solvents and provides a rejection for the first compound which is greater than the rejection for the second compound.
PCT/GB2015/052287 describes the preparation of non-naturally occurring defined monomer sequence polymers, in which two or more monomers having different backbone and/or side chain moieties are iteratively coupled to one another, with the excess unreacted monomers being separated from the growing polymer by membrane diafiltration processes.
However, the preparation of defined monomer sequence polymers using membrane diafiltration techniques has inherent drawbacks. In order for such processes to be effective, the monomers that are iteratively coupled to the growing polymer must have a molecular weight low enough to allow excess unreacted monomers to be permeated through the membrane, thereby allowing the growing polymer to be isolated and/or purified. Problems arise when it is desirable to couple bulky monomers, the excess quantities of which do not readily permeate through the membrane. Selecting a membrane with a larger molecular weight cut-off (MWCO) can often aggravate the problem, and may lead to quantities of the growing polymer being undesirably lost in the permeate.
The present invention was devised with the foregoing in mind.